1. Field of the Invention
The present invention relates to a code symbol reading apparatus, and more particularly, to a code symbol reading apparatus for infallibly and easily reading a code symbol of a commercial product in a wide range including peripheral positions of the apparatus.
2. Discussion of Related Art
As a code symbol reading apparatus which optically reads an image of a code symbol such as a barcode attached to a product and obtains a product code from the read image, an apparatus using an area image sensor such as a CCD image sensor is known. This code symbol reading apparatus extracts image information of a barcode as a subject from image data obtained with the CCD image sensor, analyzes the obtained image information and obtains code information of the barcode, i.e., a product code.
FIG. 4A is a cross-sectional view schematically showing the structure of a conventional code symbol reading apparatus. The code symbol reading apparatus 200 has an approximately rectangular parallelepiped case 210 formed with a front cover member 211 which is provided on the image sensing area F side where a product is placed and which has an image sensing opening 213, and a back cover member 212 which is engaged with the front cover member 211 and which is provided on the side opposite to the image sensing area side.
An image sensing device 220 to take image of the image sensing area F, an illumination board 230 to illuminate the image sensing area F and an image processing board 240 are provided in the case 210. The image sensing device 220 image-senses a barcode attached to a product, and the image processing board 240 analyzes image data of the image-sensed barcode and outputs product data.
The image sensing device 220 has an image sensing board 221 including a CCD image sensor (not shown) as an area image sensor and a driving circuit (not shown) for the CCD image sensor, and an lens 222 to form an image of the image sensing area F on the CCD image sensor. The illumination board 239 is provided with an image sensing opening 232 of the lens 222 in its central portion. Plural LEDs (Light Emitting Diodes) 231 to emit illumination light to the image sensing area F are provided around the opening 232. The image processing board 240 extracts a barcode portion from image data of a product outputted from the image sensing board 221, analyses the barcode data and obtains a product code.
In this code symbol reading apparatus 200, a front surface 211a is provided in a position closest to the image sensing area F side in the front cover member 211 of the case. A plate type transparent cover 250 is provided so as to cover the opening 213 on the rear side of the front surface 211a. The image sensing opening 232 of the lens 222 is provided on the illumination board 230, and the LEDs 231 are provided so as to be positioned inside the opening 213 on the image sensing area F side. Light from the LEDs 231 is emitted on an illumination area I including the image sensing area F.
A barcode of a product placed in the image sensing area F is image-sensed with the image sensing device 220. Then the image processing board 240 extracts image data of the barcode from image data of the product obtained with the image sensing device 220, then analyses the image data and obtains a product code.
As a code symbol reading apparatus having such area image sensor, Japanese Published Unexamined Patent Application No. 2005-025311 discloses a barcode reader having plural LEDs to emit illumination light on a barcode symbol and a CCD on which reflected light from the barcode symbol is image-formed via a light receiving lens. In the barcode reader, the respective LED emitting portions are covered with LED caps for light diffusion. The respective LED caps have a diffusion characteristic to diffuse the light emitted from the respective LEDs so as to form a uniform illumination distribution surface on the barcode symbol.
In many cases, in the conventional code symbol reading apparatus, for the purpose of cost reduction, the lens 222 is a fixed focus lens. In this case, in accordance with barcode position in the image sensing area F, an image obtained by image sensing may become a so-called “out-of-focus” image. From a viewpoint of resolution, as shown in FIG. 4B, in such blurred image, the resolution is a maximum in a focus position P1 of the lens, and becomes lower in the vicinity of the focus position.
Upon analysis of code symbol image by the image processing board 240, when image data as the subject of processing is in a resolution equal to or lower than a predetermined value d, analysis cannot be performed and a product code cannot be outputted. Accordingly, the barcode reading range is a predetermined area (depth of field A1) between a lens side limit Pc1 and a limit Pf1 opposite to the lens side positioned with the focus position P1 therebetween. Generally, as the depth of field A1, the focal distance of the lens 222 or the like is selected such that a product can be easily placed. That is, the depth of field A1 is selected for the optimum placement of product in the focus position P1.
In an area out of the depth of field A1, an outer area B1 of the limit Pf1 opposite to the lens side and an inner area C1 from the inside of the lens side limit Pc1 to the surface of the front cover member 211, as the resolution of image data is low, the barcode of a product placed in these areas cannot be read. On the lens side limit Pc1 side, the lens side limit Pc1 of the depth of filed A1 is away from the case 210. When a product is placed in a position closer to the case 210 than the lens side limit Pc1, the barcode of the product cannot be analyzed.
When a barcode attached to the product cannot be read with such code symbol reading apparatus, an operator places the product in the image sensing area of the code symbol reading apparatus again to perform barcode reading. However, in the conventional code symbol reading apparatus, the depth of field A1 of the image sensing device in the image sensing area is not clear. Accordingly, in some cases the operator cannot place the product in the depth of field A1 of the image sensing device again, and reading cannot be performed.
As long as the apparatus can infallibly perform barcode reading when the product is placed in a position close to the case of the code symbol reading apparatus in such retry, barcode reading can be performed only by allowing the operator to place the product in a position in close vicinity to the code symbol reading apparatus. Accordingly, a code symbol reading apparatus which infallibly performs barcode reading when a product is placed in a position close to the case of the apparatus is needed.